1. Field of the Invention
The present invention relates to a double-joint type suspension system for use with a steerable wheel of a vehicle, and more particularly to a double-joint type suspension system for a steerable wheel in which behavior of an axle in a steered direction at the beginning of steering is made smooth.
2. Description of the Related Art
In the field of a suspension system for a vehicle, as a suspension type for use with a steerable wheel, a so-called double-joint type suspension system is well known, in which two suspension arms are connected to a knuckle arm with two ball joints, respectively.
In the conventional double-joint type suspension system, it has been proposed that an elastic characteristic of a vehicle body side bushing of one suspension arm in which an angle between such suspension arm and a longitudinal axis of the vehicle is smaller than an angle between the other suspension arm and the longitudinal axis be made nonlinear in order to reduce a drag on steering. See, for example, JP4-238707A.
In the typical double-joint type suspension system, the ball joints connecting the suspension arms and the knuckle arm are configured to have some friction torques (friction drags) in order to provide a response (i.e. reaction force) to a driver when steering. However, if such friction torques of the ball joints are excessive, then those act as drags on steering when the driver steers, especially at the beginning of steering.
In the conventional double-joint type suspension system, because no attention is paid to such drags on steering generated in wheel side ball joints of the suspension arms and therefore those are not adequately reduced, those may give to the driver of the vehicle the impression that smoothness of the behavior of the axle in the steered direction is not enough, especially at the beginning of steering.